1. Field of the Invention
The present invention relates to an image forming apparatus, and specifically relates to an image forming apparatus that prevents image quality deterioration caused by condensation of water vapor generated from sheets when fixing a toner image thereto.
2. Description of the Related Art
Conventionally, in electrographic image forming apparatuses such as copiers, printers and facsimile machines, when forming an image on a sheet, first, a toner image formed in an image forming unit is transferred to a sheet. Subsequently, the toner image is fixed to the sheet by means of heating and pressurization using a fixing unit, thereby forming an image on the sheet.
Example of such image forming apparatus include an image forming apparatus including a re-transport unit for reversing a sheet with a toner image fixed to one side thereof and transporting the sheet to an image forming unit again. In two-side printing for forming an image on two sides of a sheet, a sheet with a toner image fixed to one side thereof is reversed by the re-transport unit, and is transported again to the image forming unit, thereby forming an image on the back side of the sheet.
Examples of the fixing unit include a thermal pressure fixing-type fixing unit including a fixing roller and a pressure roller, the fixing unit applying heat and pressure to a sheet simultaneously by means of the fixing roller and the pressure roller, thereby fixing a toner image to the sheet. In the case of such fixing unit, when fixing a toner image to a sheet, a considerable amount of heat is applied from the fixing roller, which heats the sheet, to the sheet. Accordingly, in the toner image fixing, moisture contained in the sheet evaporates, generating water vapor.
When water vapor is generated, if the temperature of the image forming apparatus body subsequently becomes relatively low, water vapor may condense in a sheet transport path. Upon occurrence of condensation, when a sheet passes through the sheet transport path, droplets adhere to the sheet. In order to prevent condensation, conventional image forming apparatuses include a fixing unit with an enhanced air tightness to absorb water vapor within the fixing unit. Another example of a conventional image forming apparatus is configured to discharge generated water vapor to the outside of the apparatus via a louver provided at an upper portion of the image forming apparatus (see Japanese Patent Application Laid-Open No. H08-254938).
In conventional image forming apparatuses, with an increase in speed in recent years, an amount of heat transferred from the fixing roller to a sheet is also increased, resulting in an increase in an amount of generated water vapor itself. The conventional configuration that enhances the air tightness of the fixing unit, thereby absorbing water vapor within the fixing unit has a limit on the water vapor absorption, causing difficulty in water vapor collection.
When water vapor is insufficiently discharged and collected, if the temperature of the image forming apparatus body becomes relatively low, for example, in two-side printing, water vapor condenses on a guide member that guides a reversed sheet. In recent years, for downsizing and enhanced two-side printing productivity of the image forming apparatuses, a switchback roller pair provided in a re-transport unit to reverse and transport a sheet is arranged in the vicinity of the fixing unit in some cases. In this case, water vapor condenses also on the surface of the switchback roller pair.
Upon occurrence of condensation, for example, when two-side printing is performed in a condition in which the image forming apparatus body is not sufficiently warmed up, like in the case of a cold start, water vapor condensing on the guide member and the switchback roller pair adheres to a sheet, which is to be reversed and transported, in the form of droplets. When droplets adhere to a sheet, the electric resistance value of the part of the sheet surface to which the droplets adhere is lowered compared to the surrounding part of the sheet surface to which no droplets adhere, which may cause image dropouts during transfer of a toner image in a transfer unit.